Generally, in an inverter, not only switching noise but also switching loss occurs due to the switching operations of switching elements configuring the inverter. In controlling the inverter, it is preferable to reduce both the switching noise and the switching loss. However, because reducing the noise and reducing the switching loss are conflicting requirements for the inverter, it is not easy to realize both of them.
In a related-art inverter controller, an amount of produced noise and an amount of switching loss in the inverter is adjusted by monitoring an amount of produced heat of a machine in which the inverter is installed and selecting resistance values of gate resistances which are connected to the switching elements of the inverter based on the amount of the produced heat (for example, refer to JP-A-2008-278584).
In a related-art air conditioning machine in which an inverter is installed, in the case of performing heating operation when the temperature of the outside air is low (low outside air temperature heating), a high heating ability is necessary. Because a compressor in a refrigerating cycle operates with a maximum number of revolutions, the motor current becomes the largest. However, because cooling fins are usually attached to inverter driving elements, and forced air-cooling is carried out by an outdoor unit fan, the temperature of the inverter driving elements is suppressed from rising even if the motor current is the largest. Therefore, the amount of switching loss in the inverter driving elements cannot be correctly obtained from a detected temperature of the switching elements. Therefore, even if the inverter controller as described in JP-A-2008-278584, which selects the resistance values of the gate resistances based only on the amount of produced heat of the machine to which the inverter is installed, is applied to an air conditioning machine, there is a problem that controlling the inverter with high efficiency cannot be realized.
Further, in an air conditioning machine, the amount of noise produced by the inverter varies depending on operating states. For example, when the outside air temperature is low at the time of cooling operation, when the air conditioning machine operates in a light load state such as a dry operation, or when a high voltage is supplied (direct current bus voltage is high), produced noise tends to increase. Therefore, in the related-art air conditioning machine, because it is necessary to suppress the amount of produced noise to be equal to or lower than a standard value that is regulated by the laws, switching time of the inverter driving elements, which is a main factor of noise production, is set to be a fixed large value. However, when the switching time is adjusted to be a fixed large value, if the air conditioning machine operates in an overload state, the switching loss becomes larger, thereby reducing air-conditioning ability of the air conditioning machine and the efficiency of controlling the inverter. In response to the increase of the switching loss, it becomes necessary to further enlarge the cooling fins of the inverter. Further, when a noise design is performed by setting a design value with an enough margin from the standard value regulated by the laws, there are problems that a size of a component for noise suppression increases, the number of components increases, and cost increases.